Digital television (DTV) networks are comprised of a plurality of transmitters, each broadcasting the same signal using multiple frequencies or a single frequency (single frequency network). As the number of transmitters grows, there is an increased desire to be able to identify the transmitter of origin for each signal received. Transmitter identification will enable broadcasting authorities to identify illegal or improperly operating transmitters. Moreover, transmitter identification can also be used to tune various transmitters in a single frequency network to minimize the effects of multi-path interference. Multi-path interference is caused by the destructive interference of several different transmissions originating from different transmitters and/or caused by the reflection of transmissions. FIG. 1 illustrates a single-frequency digital-television network 1, including three transmitters 2, 3 and 4 with transmission ranges 6, 7 and 8, respectively. With reference to the overlap area, designated by reference numeral 9, a receiver positioned therein would receive a transmitted signal comprised of direct transmitted signals 12, 13 and 14 from transmitters 2, 3 and 4, respectively, plus reflected transmitted signal 16 from transmitter 2. The reflected transmitted signal 16 resulting from a reflection off of a large obstacle 17, e.g. a large building. Unfortunately, the various components of the transmitted signal may not all be in phase, resulting in undesired effects. The effects of multi-path interference to DTV signals include a degradation in the television picture and sound. In analog television, multi-path interference causes “ghost” images.
U.S. Pat. Nos. 6,075,823 issued Jun. 13, 2000 to Hideaki Sonoda; 6,122,015 issued Sep. 19, 2000 to Al-Dhahir et al; 6,128,337 issued Oct. 3, 2000 to Schipper et al; 6,304,299 issued Oct. 16, 2001 to Frey et al; 6,437,832 issued Aug. 20, 2002 to Grabb et al; and 6,501,804 issued Dec. 31, 2000 issued to Rudolph et al disclose various solutions to overcoming the problem of multi-path interference. In general, the systems disclosed in the aforementioned references compare a transmitted test signal including noise with a reference signal, and construct a filter in accordance with the results of the comparison to remove noise from transmitted digital television signals. Unfortunately, none of the prior art references provide an identification signal for each transmitter, nor do they provide a system for tuning the entire network. Each of the aforementioned systems requires a complicated filtering circuit to be installed in every receiver in the system, which greatly increase the cost to the operator, and therefore the consumer.
Conventional Global Positioning Systems (GPS) do not work well inside buildings, due to the weak field strength and the high frequency of the GPS signal. In contrast to the GPS signals, the DTV signals are received from transmitters at relatively short distances, and the broadcast transmitters operate at levels up to megawatts of effective radiated power (ERP). Since the locations of DTV transmitters in DTV networks are known, it is possible to locate the position of a receiver, when the DTV signals from multiple DTV transmitters are successfully received and identified. Moreover, the RF frequency of the DTV signal is much lower than that of the GPS, which makes it easier for the DTV signal to penetrate buildings and other objects. Accordingly, there is often sufficient field strength to permit DTV signal reception and position location even inside buildings. Furthermore, the wide bandwidth of the DTV signal help to improve the accuracy of the position location.
An object of the present invention is to overcome the shortcomings of the prior art by providing a transmitter identification system that can be used to identify the transmissions, direct or redirected, from various transmitters.
Another object of the present invention is to provide timing information relating to the transmissions from known transmitters, which can be used to tune the transmitters in a network to minimize the effects of multi-path interference.
Another object of the present invention is to provide receiver location information, based on the relative position of the receiver to a plurality of transmitters.
Another object of the present invention is to provide a robust data transmission system, with a relatively large coverage area, based on the modulation of the transmitter identification sequences.